The present invention relates to a correlation calculation process execution method, a control circuit, a signal processing circuit, and a position calculation device.
The global positioning system (GPS) is widely known as a positioning system utilizing a positioning signal. The GPS is utilized for a position calculation device provided in a portable telephone, a car navigation system, and the like. A GPS receiver locates its current position by calculating four parameters (i.e., three-dimensional coordinate values that indicate the position of the GPS receiver and a clock error) based on information such as the positions of a plurality of GPS satellites and the pseudo-range between each GPS satellite and the GPS receiver (i.e., positioning calculations).
A GPS receiver has been known that performs an intermittent positioning operation in which a positioning period and a suspension period are repeated in order to reduce power consumption. In the suspension period, the operations of a frequency conversion section that down-converts the received signal into an intermediate-frequency (IF) signal and a signal processing section that performs correlation calculations on the received signal and a code replica are suspended (see JP-A-2001-42023, for example).
A GPS receiver may have a plurality of positioning modes corresponding to the reception environments such as an outdoor environment and an indoor environment. For example, the correlation value is calculated by averaging the correlation values in the outdoor environment mode since the signal strength is relatively high. On the other hand, the correlation value is calculated by integrating the correlation values in the indoor environment mode since the signal strength is low. Specifically, since the process that calculates the correlation value differs corresponding to the positioning mode, the observation target time (measurement time) of measurement information to be calculated differs even if the target period is the same. When executing a plurality of positioning modes in parallel and performing positioning calculations using the measurement information calculated in each positioning mode, a process that corrects the measurement information so that the measurement time coincides with a given time (hereinafter referred to as “propagation process”) is performed. However, since the measurement information is modified by the propagation process, the positioning accuracy deteriorates. The degree of deterioration increases as the difference between the given time and the measurement time increases.
When executing a plurality of positioning modes in parallel during the intermittent positioning operation, it is necessary to control the operation while taking into consideration the relationship between the target period in each positioning mode and the positioning/suspension period of the intermittent positioning operation. Specifically, since the measurement time of the measurement information changes due to the difference in the positioning/suspension period included in the correlation calculation target period, the difference in the measurement time of the measurement information calculated in each positioning mode increases. As a result, a positioning error due to the propagation process increases.